This invention generally relates to detection of conditions involving a risk of ice formation on the surface of aircraft in movement, based on temperature measurement. More specifically the invention comprises both a method and an apparatus in various embodiments, intended to avoid the problems represented by such ice formation.
When there is referred to aircraft here, there is in the first instance the question of airplanes and secondly other forms of aircraft, including helicopters, which also to a high degree can be subject to icing. In particular icing on the helicopter rotor can be very dangerous.
Previously there are many proposals directed to the detection of ice layers being deposited on the surface of aircraft. Examples of known methods of interest in this respect, are U.S. Pat. No. 5,521,584 and perhaps in particular U.S. Pat. No. 5,313,202.
U.S. Pat. No. 5,313,202
The patent specification takes as a starting point that the aircraft part collides with undercooled water drops existing in the air space where the part is moving. The collision results in attachment of the water drops to the aircraft part and the drops are converted into ice as a consequence of this collision. When the water freezes to ice energy is liberated in the form of heat so that the ice will have a temperature higher than the environment while the ice is formed. Ice is formed at 0xc2x0 C. This region of the aircraft part that is being covered with ice or that has been covered with ice still not being completely frozen out, therefore will have a temperature that is higher than the environment and accordingly also higher than the regions still not being covered by ice. This difference in temperature, between for example the leading edge of a helicopter rotor being in a process where ice is built upxe2x80x94and portions of the same rotor where still no ice has attached, according to the patent specification can be recorded by means of a particular contact-free detector that records the difference in infrared radiation from the regions with ice in relation to the radiation from the ice-free regions.
Nowhere in the patent specification it is mentioned that freezing can commence in front of the aircraft part, and the sensitivity of an IR detector as shown, will not be suitable in order to measure the influences on the aircraft part from temperature differences due to freezing of very small droplets of undercooled water in the air space outside the actual aircraft partxe2x80x94i.e. droplets being too small to be able to attach to the aircraft part by the freezing-out. Therefore the solution can not be employed for warning about any risk of icing.
U.S. Pat. No. 5,521,584
This known method is intended for recording that ice has been formed or is during formation, in particular on airfoils when the aircraft stands on the ground. The purpose is to save time-consuming inspection in order to decide whether or not the aircraft has to be deiced, before take off.
The functioning of the method is based upon the fact that ice has an insulating effect: After having been high in the air aircraft and the contents of fuel tanks, being often localized in the airfoils, may have been cooled to below the freezing point of ice. If there is mild air with a high moisture content at the place where the aircraft has landed, the moisture can condense and freeze to ice on the airfoil at the regions concerned. At selected places where there can be a risk of icing, special sensors are attached to be able to measure the heat flow from the moist, warm air to the cooled aircraft part. As the aircraft part is covered by an increasingly thick ice layer, the heat flow will decrease because of the insulation effect of the ice. Heat flow and temperature picture make it possible to calculate the tickness of the ice being possibly formed. Thus, the pilot on the background of the measurements can decide in a simple manner whether or not deicing is required.
This form of icing is not of interest with respect to an aircraft during flight, and accordingly this known method in the form shown can not be employed for recording ice formation as a result of collision with undercooled rain in the air during flight, and still less be able to warn about any risk of such icing.
The patent specification and claims do not contain any indication to the effect that the inventor has contemplated this possibility.
Thus, methods being known hitherto have not been directed to providing a pre-warning regarding risk of icing being present. When an ice layer has started to be deposited, there can already exist risk factors which aircraft pilots are to a highest degree interested in avoiding. Accordingly, there is a great need for means for detecting conditions involving a risk of ice formation, before such ice formation is initiated. This invention therefore is directed to this task, and is based on temperature measurements as are also known methods for detecting an ice coating having actually been deposited.
In their broadest aspect the novel and specific features of the invention in the first place consist in that the surface temperature is measured with temperature measuring elements disposed in at least two surface points having different air pressures, that a recording of the change of the temperature difference with time between the points, is taken as a basis for the detection and a corresponding indication being presented to the pilot or crew of the aircraft, and that at least one temperature measuring element gives a measure of absolute temperature, that provides a check of the validity of the detection and the indication when the absolute temperature is between 0xc2x0 C. and xe2x88x9255xc2x0 C.
In addition to the main features just recited regarding the method according to the invention, there are stated further and subordinate method features in the claims. Moreover the invention comprises an apparatus for detection of conditions involving a risk of ice formation on the surface of the aircraft in movement, based on temperature measurement, as also stated in the claims.
In technical terms it is clear that the above recording of temperature difference/time changes, will be performed by means of a computer and that this after a suitable signal processing will deliver control or output signals to for example an instrument panel that can be observed by the aircraft crew, for indications, alarm or warning to the effect that the conditions at one or more surface portions of the aircraft involve risk of ice formation. Accordingly, necessary action can be taken in due time, for example by adequate maneuvering or activation of deicing equipment.
By means of the invention it is possible for example for a passenger airplane pilot to follow the development of the relevant temperature relationships through several minutes before the instant when icing will start. For aircraft with a velocity of several hundred kilometers per hour this means that warning about risk of ice formation can be indicated several tens of kilometers beforehand. The significance of this can not be over-estimated.
The invention is based in part on a new understanding regarding the mechanisms or processes that occur immediately adjacent to the surfaces of aircraft in movement, and in the following description these relationships will be discussed further. In general regarding meterological relationships of interest in this connection, reference is here made to the book xe2x80x9cVxc3xa6r og klima i fargerxe2x80x9d (title in English: Weather and climat in colours) by Petter Dannevig and Svante Bodin, H. Aschehoug and Co., Oslo 1978, see in particular page 105 pp in the book.
When an aircraft moves through the air, there will be local variations in the relative velocity between the air molecules and the various parts of the surface of the aircraftxe2x80x94depending on the shape and the manner of its movement. In particular the leading edge of the airfoil and tail rudder profiles as well as the propeller, and on helicopters: the main rotor and the tail rotor, are regions of interest. In technical terminology these regions are designated stagnation regions, i.e. the regions of the aircraft being oriented at right angles to or close to right angles to the direction of movement. Here there will be a damming up of air molecules that will result in a pressure increase in relation to the stationary ambient pressure,xe2x80x94in a popular sense because of the fact that the air molecules in this region need time in order to be accelerated from standstill to approximately the velocity of the aircraft. This pressure increase will lead to an adiabatic temperature increase that in dry air at for example 500 km/h theoretically will be about 10xc2x0 C.
The frictional work between the air and the surface of the aircraft also contributes to an increase in temperature. Near the stagnation region this contribution will be modest, but in the regions where the relative velocity corresponds to the velocity of the airplane, the temperature increase due to the frictional work will be of the same order of magnitude as from the pressure damming. During normal flight there will also be a higher pressure on the underside of the airfoils than on the upper side. This pressure difference will also result in a corresponding temperature difference. The various contributions that have influence on the temperature, result in a temperature profile that can be mapped by recording the temperature distribution on the surface of the aircraft. The results from the various contributions mentioned above, will be influenced by the flight conditions of the aircraft, i.e. by the velocity, by the angle of attack at the airfoils, by the motor power etc. For airplanes with fixed airfoils also the use of slots, flaps and so forth will have an influence on the temperature profile.
If the air contains water vapor or gases having other physical properties than air, the temperature profile will be altered in relation to the profile for clean, dry air. Pressure changes and frictional resistance will also be the cause of turbulence in the space around the aircraft. If there are undercooled water drops in the air, the freezing of these may be initiated in consequence of the turbulence, the quick temperature changes and the pressure damming in front of the aircraft. Formation of ice from the water droplets normally takes place at about 0xc2x0 C. (the freezing temperature is somewhat dependent on the pressure), and at the phase transition from water to ice, there will be liberated a large amount of energy per unit of water content. If the water is undercooled portions of this energy will be consumed for heating the water, but some of the energy will also gradually be supplied to the air surrounding and between the water droplets. Freezing of undercooled water therefore will result in an increase of the temperature in relation to the temperature before the start of the process.
At the outsets before freezing-out commences, there can not be any significant temperature differences between the water droplets and the surrounding air. When freezing starts the energy from the phase transition will be liberated in the water, which therefore rather instantaneously will get an increase in temperature up towards 0xc2x0 C. In this way there will be established during a short time a temperature difference between the water that freezes and the surrounding air, and there will be initiated a heat flow from the water to the air. The heat flux between the water and the air will be a function of the size of the water droplets and of their number per volume unitxe2x80x94as well as of the density (pressure) of the air at the location.
If the undercooled water droplets are small and if the amount of these is limited, the transition heat may as a whole be taken up by the water droplets and the surrounding air. If the droplets are larger than a critical minimum size, possibly with a sufficiently high density of droplets, it is not possible to consume or distribute all freezing energy sufficiently quickly. Therefore the droplets will be hit by the stagnation surfaces before they have become ice; they will be of a xe2x80x9cslurry consistencyxe2x80x9d and can now freeze to compact ice at contact with the cooled parts of the flow profile, or at contact with ice that has already attached to the aircraft.
When icing occurs during flight the aggregation always commences at the stagnation regions and the ice then grows outwards from this region. When icing has started the shape of the profile can be changed as the thickness of the ice increases, and the airfoil and tail profiles can be altered so that both the carrying capacity and the maneuvring properties will be highly depreciated or destroyed. Besides with the altered shape there can also be formed other and new stagnation surfaces and regions where the ice can also build up.
Undercooling and freezing-out of ice can comprise specific mechanisms or processes. It is a common notion that the freezing point of water and the melting point of ice has the same temperature value. This however, is only valid when large water surfaces are concerned. When there is a question of very small droplets, the freezing must be initiated. In a way this is analogous to the fact that condensation nuclei are required in order to form water drops from vapor. Without the presence of freezing nuclei that can start the freezing process and without kinetic or thermal disturbances, the water can remain undercooled during a long time, in particular if the air temperature is not lower than about xe2x88x9212xc2x0 C. Exceptionally there has been found undercooled water down to about xe2x88x9240xc2x0 C. Sudden temperature or pressure changes, for example as a result of turbulence from an airplane moving through the undercooled water droplets, can trigger the freezing process. It is believed that there is less need for triggering the freezing-out the more undercooled the droplets arexe2x80x94and the larger they are. The end temperature can be calculated if the air pressure, the amount of undercooled water and the initial or starting temperature are known. Conversely if the air pressure, the initial and the end temperatures are known, it is possible to determine the amount of undercooled water. Calculations have shown that if there are only small amounts of undercooled water, all the heat liberated will not be sufficient for bringing the end temperature locally higher than 0xc2x0 C. This means that there will not be any icing risk for airplanes, since the water droplets will be transformed into ice before the airplane collides with them.
If on the other hand the amount of undercooled water increases, it will take a longer time to bring away the heat liberated, and all the water will not freeze to ice before the airplane hits the droplets. The droplets will now be in a mixture of ice and water, i.e. a type of adhesive slurry that can attach to airfoils, tail surfaces etc. and icing conditions are present. If airfoil surfaces, tail surfaces and so forth beforehand have temperatures below 0xc2x0 C., they can take up residual heat so that the water freezes to ice and will be well attached to the surfaces concerned. Note in this connection that the formation of ice from a given amount of water requires that the same energy is transported away from the water as what is needed for cooling the same amount of water by about 80xc2x0 C.